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Thread: Question on USC to UMP conversion

  1. #21
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    Should be able to use the UMP lower guts without having to switch out.
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    Op, I've got a Bostic fused G36 which in my opinion the only way to go on that rifle. Cost is higher but the stock block conversion is just off - length of pull too long, rifle looks and shoots weird to me so I'm glad I did that one. I will say I'd never use that gun in a serious SHTF scenario - never bad a problem but cutting and gluing the polymer housing just creates another potential point of failure in my mind (perhaps not in reality but the builders have voiced some cautions as well).

    I did choose to go stock block conversion on the UMP and it's great. I'm not a purest HK collector and some won't do anything but a fuse job but the difference on the USC is pretty minimal and to me, the cost benefit of the fuse job isnt there, it looks right and shoulders great. And, I don't ever worry about the fuse job. Best, Matt

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    Quote Originally Posted by AmericaNL View Post
    Hi, all. I will soon be starting on a USC-to-UMP (.45) conversion. I will do the "basic" stuff myself (rail, change the guts of the lower, stock block, etc), but will be working with a local gunsmith for the machining bits. I do have a quick question for those more experienced here.

    When changing the lower, do I need the full lower (link to hkpart) or can I get a "stripped pistol grip" lower (link to hkparts), UMP lower backplate (link to hkparts) and stock block (link to hkparts)?

    Thanks in advance for any insight you can offer!
    Hi AmericaNL,

    I bought some of my parts from Gunbroker and the stock block from HK parts. I bought a complete FBI lower and the folding stock/pin/stub from GB and the stock block from HK parts. From what I've read you can buy the stripped lower and change out the parts but I'm not 100% sure about that.

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    Quote Originally Posted by meh92 View Post
    Good question. I was wondering the same thing. Will the guts of my USC drop in to an FBI 2-position lower?
    So I recently bought a USC with a 2 position FBI lower-the original owner did just that and swapped out the parts from his USC-the selector switch is pretty slopy and sometimes gets hung up. I'm concerned that the parts were not replaced correctlly--I'm shocked at how flimsy the selector switch is- no positive detent as on mP5s. Is this normal for the polymer guns?

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    Quote Originally Posted by ATL_Matt View Post
    Op, I've got a Bostic fused G36 which in my opinion the only way to go on that rifle. Cost is higher but the stock block conversion is just off - length of pull too long, rifle looks and shoots weird to me so I'm glad I did that one. I will say I'd never use that gun in a serious SHTF scenario - never bad a problem but cutting and gluing the polymer housing just creates another potential point of failure in my mind (perhaps not in reality but the builders have voiced some cautions as well).

    I did choose to go stock block conversion on the UMP and it's great. I'm not a purest HK collector and some won't do anything but a fuse job but the difference on the USC is pretty minimal and to me, the cost benefit of the fuse job isnt there, it looks right and shoulders great. And, I don't ever worry about the fuse job. Best, Matt
    Very confusing post.

    Matt were you quoting someone from a different thread? If I were to paraphrase the above post: the top paragraph says "stock block bad, go fuze!" then "fuze may break". The second paragraph says "stock block great! fuze job 'meh' "
    Last edited by Mr.Maim; 11-08-2016 at 03:43 AM.

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    Quote Originally Posted by Cjude View Post
    So I recently bought a USC with a 2 position FBI lower-the original owner did just that and swapped out the parts from his USC-the selector switch is pretty slopy and sometimes gets hung up. I'm concerned that the parts were not replaced correctlly--I'm shocked at how flimsy the selector switch is- no positive detent as on mP5s. Is this normal for the polymer guns?
    No it is not normal. The selector in the UMP is solid with positive detents.

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    Quote Originally Posted by cramer View Post
    You will need to get the wider UMP mag catch pin (along with the mag catch), the USC is shorter. Also, grab an additional trigger housing pin. The UMP has an extra hole.

    You do not need the UMP backplate if you are using the HDPS stock block. It takes the place of it.
    To come back to this, I have read in a couple places that people have simply enlarged the mag well on a USC lower to accept UMP mags, modified the receiver and slapped on a Hera arms stock block. This sounds like it may be a reasonable way to go, but does anyone have pictures of a WELL DONE mag well modification?

  9. #28
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    Default Question on USC to UMP conversion

    You will have to make and attach new sides to the magazine opening. The UMP mag is so much wider that opening it enough to fit it will remove all of the sides of the mag opening.


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